JavaTM 2 Platform
Standard Ed. 5.0

java.util.concurrent
Class CopyOnWriteArrayList<E>

java.lang.Object
  extended by java.util.concurrent.CopyOnWriteArrayList<E>
Type Parameters:
E - the type of elements held in this collection
All Implemented Interfaces:
Serializable, Cloneable, Iterable<E>, Collection<E>, List<E>, RandomAccess

public class CopyOnWriteArrayList<E>
extends Object
implements List<E>, RandomAccess, Cloneable, Serializable

A thread-safe variant of ArrayList in which all mutative operations (add, set, and so on) are implemented by making a fresh copy of the underlying array.

This is ordinarily too costly, but may be more efficient than alternatives when traversal operations vastly outnumber mutations, and is useful when you cannot or don't want to synchronize traversals, yet need to preclude interference among concurrent threads. The "snapshot" style iterator method uses a reference to the state of the array at the point that the iterator was created. This array never changes during the lifetime of the iterator, so interference is impossible and the iterator is guaranteed not to throw ConcurrentModificationException. The iterator will not reflect additions, removals, or changes to the list since the iterator was created. Element-changing operations on iterators themselves (remove, set, and add) are not supported. These methods throw UnsupportedOperationException.

This class is a member of the Java Collections Framework.

Since:
1.5
See Also:
Serialized Form

Constructor Summary
CopyOnWriteArrayList()
          Creates an empty list.
CopyOnWriteArrayList(Collection<? extends E> c)
          Creates a list containing the elements of the specified Collection, in the order they are returned by the Collection's iterator.
CopyOnWriteArrayList(E[] toCopyIn)
          Create a new CopyOnWriteArrayList holding a copy of given array.
 
Method Summary
 boolean add(E element)
          Appends the specified element to the end of this list.
 void add(int index, E element)
          Inserts the specified element at the specified position in this list.
 boolean addAll(Collection<? extends E> c)
          Appends all of the elements in the specified Collection to the end of this list, in the order that they are returned by the specified Collection's Iterator.
 boolean addAll(int index, Collection<? extends E> c)
          Inserts all of the elements in the specified Collection into this list, starting at the specified position.
 int addAllAbsent(Collection<? extends E> c)
          Appends all of the elements in the specified Collection that are not already contained in this list, to the end of this list, in the order that they are returned by the specified Collection's Iterator.
 boolean addIfAbsent(E element)
          Append the element if not present.
 void clear()
          Removes all of the elements from this list.
 Object clone()
          Returns a shallow copy of this list.
 boolean contains(Object elem)
          Returns true if this list contains the specified element.
 boolean containsAll(Collection<?> c)
          Returns true if this Collection contains all of the elements in the specified Collection.
 boolean equals(Object o)
          Compares the specified Object with this List for equality.
 E get(int index)
          Returns the element at the specified position in this list.
 int hashCode()
          Returns the hash code value for this List.
 int indexOf(E elem, int index)
          Searches for the first occurrence of the given argument, beginning the search at index, and testing for equality using the equals method.
 int indexOf(Object elem)
          Searches for the first occurrence of the given argument, testing for equality using the equals method.
 boolean isEmpty()
          Tests if this list has no elements.
 Iterator<E> iterator()
          Returns an Iterator over the elements contained in this collection.
 int lastIndexOf(E elem, int index)
          Searches backwards for the specified object, starting from the specified index, and returns an index to it.
 int lastIndexOf(Object elem)
          Returns the index of the last occurrence of the specified object in this list.
 ListIterator<E> listIterator()
          Returns an Iterator of the elements in this List (in proper sequence).
 ListIterator<E> listIterator(int index)
          Returns a ListIterator of the elements in this List (in proper sequence), starting at the specified position in the List.
 E remove(int index)
          Removes the element at the specified position in this list.
 boolean remove(Object o)
          Removes a single instance of the specified element from this list, if it is present (optional operation).
 boolean removeAll(Collection<?> c)
          Removes from this Collection all of its elements that are contained in the specified Collection.
 boolean retainAll(Collection<?> c)
          Retains only the elements in this Collection that are contained in the specified Collection (optional operation).
 E set(int index, E element)
          Replaces the element at the specified position in this list with the specified element.
 int size()
          Returns the number of elements in this list.
 List<E> subList(int fromIndex, int toIndex)
          Returns a view of the portion of this List between fromIndex, inclusive, and toIndex, exclusive.
 Object[] toArray()
          Returns an array containing all of the elements in this list in the correct order.
<T> T[]
toArray(T[] a)
          Returns an array containing all of the elements in this list in the correct order.
 String toString()
          Returns a string representation of this Collection, containing the String representation of each element.
 
Methods inherited from class java.lang.Object
finalize, getClass, notify, notifyAll, wait, wait, wait
 

Constructor Detail

CopyOnWriteArrayList

public CopyOnWriteArrayList()
Creates an empty list.


CopyOnWriteArrayList

public CopyOnWriteArrayList(Collection<? extends E> c)
Creates a list containing the elements of the specified Collection, in the order they are returned by the Collection's iterator.

Parameters:
c - the collection of initially held elements

CopyOnWriteArrayList

public CopyOnWriteArrayList(E[] toCopyIn)
Create a new CopyOnWriteArrayList holding a copy of given array.

Parameters:
toCopyIn - the array (a copy of this array is used as the internal array)
Method Detail

size

public int size()
Returns the number of elements in this list.

Specified by:
size in interface Collection<E>
Specified by:
size in interface List<E>
Returns:
the number of elements in this list.

isEmpty

public boolean isEmpty()
Tests if this list has no elements.

Specified by:
isEmpty in interface Collection<E>
Specified by:
isEmpty in interface List<E>
Returns:
true if this list has no elements; false otherwise.

contains

public boolean contains(Object elem)
Returns true if this list contains the specified element.

Specified by:
contains in interface Collection<E>
Specified by:
contains in interface List<E>
Parameters:
elem - element whose presence in this List is to be tested.
Returns:
true if the specified element is present; false otherwise.

indexOf

public int indexOf(Object elem)
Searches for the first occurrence of the given argument, testing for equality using the equals method.

Specified by:
indexOf in interface List<E>
Parameters:
elem - an object.
Returns:
the index of the first occurrence of the argument in this list; returns -1 if the object is not found.
See Also:
Object.equals(Object)

indexOf

public int indexOf(E elem,
                   int index)
Searches for the first occurrence of the given argument, beginning the search at index, and testing for equality using the equals method.

Parameters:
elem - an object.
index - the index to start searching from.
Returns:
the index of the first occurrence of the object argument in this List at position index or later in the List; returns -1 if the object is not found.
See Also:
Object.equals(Object)

lastIndexOf

public int lastIndexOf(Object elem)
Returns the index of the last occurrence of the specified object in this list.

Specified by:
lastIndexOf in interface List<E>
Parameters:
elem - the desired element.
Returns:
the index of the last occurrence of the specified object in this list; returns -1 if the object is not found.

lastIndexOf

public int lastIndexOf(E elem,
                       int index)
Searches backwards for the specified object, starting from the specified index, and returns an index to it.

Parameters:
elem - the desired element.
index - the index to start searching from.
Returns:
the index of the last occurrence of the specified object in this List at position less than index in the List; -1 if the object is not found.

clone

public Object clone()
Returns a shallow copy of this list. (The elements themselves are not copied.)

Overrides:
clone in class Object
Returns:
a clone of this list.
See Also:
Cloneable

toArray

public Object[] toArray()
Returns an array containing all of the elements in this list in the correct order.

Specified by:
toArray in interface Collection<E>
Specified by:
toArray in interface List<E>
Returns:
an array containing all of the elements in this list in the correct order.
See Also:
Arrays.asList(Object[])

toArray

public <T> T[] toArray(T[] a)
Returns an array containing all of the elements in this list in the correct order. The runtime type of the returned array is that of the specified array. If the list fits in the specified array, it is returned therein. Otherwise, a new array is allocated with the runtime type of the specified array and the size of this list.

If the list fits in the specified array with room to spare (i.e., the array has more elements than the list), the element in the array immediately following the end of the collection is set to null. This is useful in determining the length of the list only if the caller knows that the list does not contain any null elements.

Specified by:
toArray in interface Collection<E>
Specified by:
toArray in interface List<E>
Parameters:
a - the array into which the elements of the list are to be stored, if it is big enough; otherwise, a new array of the same runtime type is allocated for this purpose.
Returns:
an array containing the elements of the list.
Throws:
ArrayStoreException - the runtime type of a is not a supertype of the runtime type of every element in this list.

get

public E get(int index)
Returns the element at the specified position in this list.

Specified by:
get in interface List<E>
Parameters:
index - index of element to return.
Returns:
the element at the specified position in this list.
Throws:
IndexOutOfBoundsException - if index is out of range (index < 0 || index >= size()).

set

public E set(int index,
             E element)
Replaces the element at the specified position in this list with the specified element.

Specified by:
set in interface List<E>
Parameters:
index - index of element to replace.
element - element to be stored at the specified position.
Returns:
the element previously at the specified position.
Throws:
IndexOutOfBoundsException - if index out of range (index < 0 || index >= size()).

add

public boolean add(E element)
Appends the specified element to the end of this list.

Specified by:
add in interface Collection<E>
Specified by:
add in interface List<E>
Parameters:
element - element to be appended to this list.
Returns:
true (as per the general contract of Collection.add).

add

public void add(int index,
                E element)
Inserts the specified element at the specified position in this list. Shifts the element currently at that position (if any) and any subsequent elements to the right (adds one to their indices).

Specified by:
add in interface List<E>
Parameters:
index - index at which the specified element is to be inserted.
element - element to be inserted.
Throws:
IndexOutOfBoundsException - if index is out of range (index < 0 || index > size()).

remove

public E remove(int index)
Removes the element at the specified position in this list. Shifts any subsequent elements to the left (subtracts one from their indices).

Specified by:
remove in interface List<E>
Parameters:
index - the index of the element to removed.
Returns:
the element that was removed from the list.
Throws:
IndexOutOfBoundsException - if index out of range (index < 0 || index >= size()).

remove

public boolean remove(Object o)
Removes a single instance of the specified element from this list, if it is present (optional operation). More formally, removes an element e such that (o==null ? e==null : o.equals(e)), if the list contains one or more such elements. Returns true if the list contained the specified element (or equivalently, if the list changed as a result of the call).

Specified by:
remove in interface Collection<E>
Specified by:
remove in interface List<E>
Parameters:
o - element to be removed from this list, if present.
Returns:
true if the list contained the specified element.

addIfAbsent

public boolean addIfAbsent(E element)
Append the element if not present.

Parameters:
element - element to be added to this Collection, if absent.
Returns:
true if added

containsAll

public boolean containsAll(Collection<?> c)
Returns true if this Collection contains all of the elements in the specified Collection.

This implementation iterates over the specified Collection, checking each element returned by the Iterator in turn to see if it's contained in this Collection. If all elements are so contained true is returned, otherwise false.

Specified by:
containsAll in interface Collection<E>
Specified by:
containsAll in interface List<E>
Parameters:
c - the collection
Returns:
true if all elements are contained
See Also:
Collection.contains(Object)

removeAll

public boolean removeAll(Collection<?> c)
Removes from this Collection all of its elements that are contained in the specified Collection. This is a particularly expensive operation in this class because of the need for an internal temporary array.

Specified by:
removeAll in interface Collection<E>
Specified by:
removeAll in interface List<E>
Parameters:
c - the collection
Returns:
true if this Collection changed as a result of the call.
See Also:
Collection.remove(Object), Collection.contains(Object)

retainAll

public boolean retainAll(Collection<?> c)
Retains only the elements in this Collection that are contained in the specified Collection (optional operation). In other words, removes from this Collection all of its elements that are not contained in the specified Collection.

Specified by:
retainAll in interface Collection<E>
Specified by:
retainAll in interface List<E>
Parameters:
c - the collection
Returns:
true if this Collection changed as a result of the call.
See Also:
Collection.remove(Object), Collection.contains(Object)

addAllAbsent

public int addAllAbsent(Collection<? extends E> c)
Appends all of the elements in the specified Collection that are not already contained in this list, to the end of this list, in the order that they are returned by the specified Collection's Iterator.

Parameters:
c - elements to be added into this list.
Returns:
the number of elements added

clear

public void clear()
Removes all of the elements from this list.

Specified by:
clear in interface Collection<E>
Specified by:
clear in interface List<E>

addAll

public boolean addAll(Collection<? extends E> c)
Appends all of the elements in the specified Collection to the end of this list, in the order that they are returned by the specified Collection's Iterator.

Specified by:
addAll in interface Collection<E>
Specified by:
addAll in interface List<E>
Parameters:
c - elements to be inserted into this list.
Returns:
true if any elements are added
See Also:
Collection.add(Object)

addAll

public boolean addAll(int index,
                      Collection<? extends E> c)
Inserts all of the elements in the specified Collection into this list, starting at the specified position. Shifts the element currently at that position (if any) and any subsequent elements to the right (increases their indices). The new elements will appear in the list in the order that they are returned by the specified Collection's iterator.

Specified by:
addAll in interface List<E>
Parameters:
index - index at which to insert first element from the specified collection.
c - elements to be inserted into this list.
Returns:
true if any elements are added
Throws:
IndexOutOfBoundsException - index out of range (index < 0 || index > size()).

toString

public String toString()
Returns a string representation of this Collection, containing the String representation of each element.

Overrides:
toString in class Object
Returns:
a string representation of the object.

equals

public boolean equals(Object o)
Compares the specified Object with this List for equality. Returns true if and only if the specified Object is also a List, both Lists have the same size, and all corresponding pairs of elements in the two Lists are equal. (Two elements e1 and e2 are equal if (e1==null ? e2==null : e1.equals(e2)).) In other words, two Lists are defined to be equal if they contain the same elements in the same order.

This implementation first checks if the specified object is this List. If so, it returns true; if not, it checks if the specified object is a List. If not, it returns false; if so, it iterates over both lists, comparing corresponding pairs of elements. If any comparison returns false, this method returns false. If either Iterator runs out of elements before the other it returns false (as the Lists are of unequal length); otherwise it returns true when the iterations complete.

Specified by:
equals in interface Collection<E>
Specified by:
equals in interface List<E>
Overrides:
equals in class Object
Parameters:
o - the Object to be compared for equality with this List.
Returns:
true if the specified Object is equal to this List.
See Also:
Object.hashCode(), Hashtable

hashCode

public int hashCode()
Returns the hash code value for this List.

This implementation uses the definition in List.hashCode().

Specified by:
hashCode in interface Collection<E>
Specified by:
hashCode in interface List<E>
Overrides:
hashCode in class Object
Returns:
the hash code
See Also:
Object.equals(java.lang.Object), Hashtable

iterator

public Iterator<E> iterator()
Returns an Iterator over the elements contained in this collection. The iterator provides a snapshot of the state of the list when the iterator was constructed. No synchronization is needed while traversing the iterator. The iterator does NOT support the remove method.

Specified by:
iterator in interface Iterable<E>
Specified by:
iterator in interface Collection<E>
Specified by:
iterator in interface List<E>
Returns:
the iterator

listIterator

public ListIterator<E> listIterator()
Returns an Iterator of the elements in this List (in proper sequence). The iterator provides a snapshot of the state of the list when the iterator was constructed. No synchronization is needed while traversing the iterator. The iterator does NOT support the remove, set, or add methods.

Specified by:
listIterator in interface List<E>
Returns:
the iterator

listIterator

public ListIterator<E> listIterator(int index)
Returns a ListIterator of the elements in this List (in proper sequence), starting at the specified position in the List. The specified index indicates the first element that would be returned by an initial call to nextElement. An initial call to previousElement would return the element with the specified index minus one. The ListIterator returned by this implementation will throw an UnsupportedOperationException in its remove, set and add methods.

Specified by:
listIterator in interface List<E>
Parameters:
index - index of first element to be returned from the ListIterator (by a call to getNext).
Returns:
the iterator
Throws:
IndexOutOfBoundsException - index is out of range (index < 0 || index > size()).

subList

public List<E> subList(int fromIndex,
                       int toIndex)
Returns a view of the portion of this List between fromIndex, inclusive, and toIndex, exclusive. The returned List is backed by this List, so changes in the returned List are reflected in this List, and vice-versa. While mutative operations are supported, they are probably not very useful for CopyOnWriteArrayLists.

The semantics of the List returned by this method become undefined if the backing list (i.e., this List) is structurally modified in any way other than via the returned List. (Structural modifications are those that change the size of the List, or otherwise perturb it in such a fashion that iterations in progress may yield incorrect results.)

Specified by:
subList in interface List<E>
Parameters:
fromIndex - low endpoint (inclusive) of the subList.
toIndex - high endpoint (exclusive) of the subList.
Returns:
a view of the specified range within this List.
Throws:
IndexOutOfBoundsException - Illegal endpoint index value (fromIndex < 0 || toIndex > size || fromIndex > toIndex).

JavaTM 2 Platform
Standard Ed. 5.0

Submit a bug or feature
For further API reference and developer documentation, see Java 2 SDK SE Developer Documentation. That documentation contains more detailed, developer-targeted descriptions, with conceptual overviews, definitions of terms, workarounds, and working code examples.

Copyright 2004 Sun Microsystems, Inc. All rights reserved. Use is subject to license terms. Also see the documentation redistribution policy.


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